1. Field of the Invention
The present invention relates to a random-block copolymer, a process for producing the same and a monofilament formed of this random-block copolymer. More specifically, the present invention relates to a random-block copolymer comprising a random copolymer segment containing a lactide unit and a caprolactone unit and a block copolymer segment containing a glycolide unit, a process for producing the same, and a monofilament which is formed of the random-block copolymer and which is appropriately used as a surgical suture.
2. Description of the Prior Art
As a random-block copolymer, polyglycolic acid (or polyglycolide), polylactic acid (or polylactide), polycaprolactone, poly(p-dioxanone). poly(trimethylene carbonate) and a copolymer thereof are known.
A multifilament suture produced from polyglycolic acid or a glycolide-lactide copolymer is in the form of a twisted yarn for providing a flexibility and a strength required because its material has a high rigidity. For this reason, bacteria tend to adhere to its surface. Further, since the filament is hard, a coating layer has to be formed on the surface to improve a tie down property. Accordingly, there is a problem that the production step becomes intricate (Japanese Patent Laid-Open No. 62,899/1973, U.S. Ser. No. 200,706).
In order to solve these drawbacks, Doddi et al produce a bioabsorbable monofilament from poly(p-dioxanone) and propose a suture which is as good as a twisted multifilament with respect to flexibility and strength (Japanese Patent Publication No. 36,785/1985, U.S. Pat. No. 4,052,988). The monofilament formed of poly(p-dioxanone) is indeed good in flexibility and strength, but has a low rate of hydrolysis. Accordingly, this monofilament is problematic in that it remains within the body for a long period of time.
Then, Bezwada et al disclose a bioabsorbable monofilament obtained from a random-block copolymer comprising a random copolymer segment of lactide and glycolide and a block polymer segment of p-dioxanone to improve flexibility, strength and further hydrolyzability of a monofilament made of poly(p-dioxanone) [Japanese Patent Laid-Open No.212,366/1992, U.S. Pat. No. 5,007,923]. It is however found that this random-block copolymer is improved in a flexibility (Young's modulus) and tensile strength but shows too high rate of hydrolysis in comparison with a monofilament formed of poly(p-dioxanone).
Further, Bezwada et al disclose a method in which a random copolymer of .epsilon.-caprolactone and glycolide is formed, and glycolide is further block-polymerized therewith to obtain a random-block copolymer (Japanese Patent Laid-Open No. 269,013/1991, U.S. Pat. No. 4,605,730). However, when the present inventors evaluated properties and the like thereof, it was found that the random-block copolymer of .epsilon.-caprolactone and glycolide has also a markedly high rate of hydrolysis.
Still further, Japanese Patent Laid-Open No. 132,638/1997 (EP-A-761712) discloses a three-component block copolymer comprising a polylactic acid segment, a poly(.epsilon.-caprolactone) segment and a glycolic acid segment, obtained by first subjecting from 20 to 1,200 parts by weight of .epsilon.-caprolactone to ring-opening polymerization in the presence of 100 parts by weight of polylactic acid containing a terminal hydroxyl group and having a weight average molecular weight of at least 2,000 and at most 500,000 and then adding from 15 to 1,200 parts by weight of glycolide during, or after the completion of, the ring-opening polymerization of the .epsilon.-caprolactone to conduct the ring-opening polymerization, this copolymer having a weight average molecular weight of from 10,000 to 1,000,000. A monofilament obtained from this three-component block copolymer has excellent strength, but is not yet satisfactory with respect to flexibility.